Typical mattresses are composed of a wire inner spring unit, two layers of cushioning materials, and two insulator pads. Generally, the insulator pads are placed within the mattress on the top and bottom of the wire inner spring unit below the cushioning material. The insulator pads serve to protect the cushioning materials from the inner spring unit and also to prevent the cushioning materials from falling into the inner spring unit.
To provide extra comfort and extended wear of mattresses, it would be desirable to provide selected zones of relatively increased firmness within the mattress. Most desirably, the zones of relatively increased firmness are located in the areas of the mattress that are subjected to maximum loading and frequent and substantial wear and use. Desirable areas of relatively increased firmness include the transverse head, center and foot. The center of the mattress is selected as a zone of extra firmness because the center must often bear the most weight during sleeping. The head and foot of the mattress are also selected as zones of extra firmness due to wear from sitting.
One known manner of providing selected zones of firmness in a mattress is to increase the thickness of the cushioning material in the area that is to be reinforced. However, increasing the cushioning materials has proved to be economically not feasible.
Another manner for providing selected zones of firmness in a mattress is to reinforce the wire inner spring unit. Reinforcing the wire inner spring unit is accomplished by increasing coil density or employing heavier spring wire in the selected areas. Increasing the coil density in certain areas has proved to be very costly. Increasing the coil density also adds substantial complexity to the manufacturing process due to the variations caused by adding firmness only in selected areas. The use of heavier spring wire for the inner spring unit adds substantial cost and may compromise other mattress components. For example, heavier spring wire in the areas of frequent use may cause the heavier wire to tear into the insulator pad and eventually into the cushioning materials.
A popular approach to providing extra firmness in a mattress is to employ two insulator pads in the areas where increased firmness is desired. The extra pad is commonly referred to as the crown pad. Use of the crown pad requires extra handling of the crown pad and also extra steps to align the crown pad. In addition, the use of an additional pad adds expense and extra inventory.
From the foregoing, it is readily seen that there is a need for a mattress with selected zones of firmness that is cost effective and easy to manufacture.